In a typical early boiler for building heating, a burner to generate heat is fired in response to detected water temperature or steam pressure in the boiler itself, depending upon boiler type. The hot water or steam generated in the boiler is circulated throughout the building to heat transfer elements (radiators). There is not individual control of heat delivery into particular zones or rooms and, typically, windows are opened for cooling. More heat is produced than is necessary and a substantial amount of the heat energy that the user pays for is wasted.
A somewhat more sophisticated boiler control system includes a thermostatic control valve at each radiator to control heat transfer from the boiler through individual radiators to each zone. Substantial heat energy is lost in this type of system as well because, since the thermostatic valves are mounted directly on the radiators, each radiator must be over-heated to heat the zone reasonably rapidly and because there is no cooperation of heating control among zones.
More advanced boiler control systems operate the boiler with consideration given to the temperatures of building zones, the outside temperature and the available rate of heat transfer from the boiler. Although such systems generally are substantially more efficient than the basic controls, a significant amount of heat energy is lost each time the boiler executes a boiler sequence, for reasons described hereinafter.
Referring to FIG. 1, a typical boiler sequence consists of the following series of steps:
(1) pre-purge
(2) ignition
(3) fire
(4) shut down
(5) post-purge
(6) stop
During Pre-Purge, the boiler burner forces air (but no fuel) through the combustion chamber of the boiler to eliminate any residual fuel that might otherwise burn explosively when ignition takes place; this usually occurs for between ten and thirty seconds. Upon Ignition, the flow of air through the combustion chamber is cut to a minimum, a fuel valve is opened, and a high voltage spark ignites the fuel-air mixture; for about ten seconds. During subsequent Firing, an air inlet is opened to a maximum and the boiler fires until the operating control is satisfied, i.e., until the temperature (water boiler) or pressure (steam boiler) has reached a predetermined temperature or pressure value; the duration of this portion of the sequence is variable, depending upon the size of the heating load involved. Shut Down, which extends for about ten seconds, closes the fuel and air valves and, during Post-Purge, the blower thereafter continues to run with the air valve opened to a maximum to force residual partially burned fuel and gases from the boiler (about 30-45 seconds). Finally, the blower shuts off (Stop).
The Pre-Purge and Post-Purge steps of the boiler cycle are critical to the safe operation of the system, but heat is lost each time the boiler is purged of already heated gases, usually by colder air. It is accordingly preferred to reduce the frequency of sequences (as outlined above) executed by the boiler for a given heat load, thereby to maximize the efficiency of the system. As a further benefit, sequence reduction substantially reduces the amount of maintenance required on the boiler.
Accordingly, it is an object of this invention to increase the efficiency of a boiler by reducing the frequency of sequences executed by the boiler for a given heat load.
Another object is to provide a method of and system for improving the efficiency of a boiler without adversely affecting space comfort.
A further object of the invention is to provide a method of and system for improving the efficiency of a boiler by utilizing the inherent overcapacity of a boiler with respect to a given load.